Recorders of physiological signals comprise an external case, carried by the patient, to which case one or more sensing electrodes are connected. The sensing electrodes are applied to the body of the patient and connected to the case at a signal input via connecting cables. The connecting cables typically include conductive wire and an insulating sheath. During frequent use, particularly with respect to recorders used for long durations, these cables are subjected to many stresses and end up being degraded. The degradation can go so far as to rupture the electric conductors internal to the cables.
The state of the connecting cables therefore must be monitored. This can be automatically carried out by making an impedance measurement and identifying changes in impedances. It is known, for example, to make such a measurement by injecting a small current on the signal input to which the cable is to be connected (the return path of the current being provided by another measurement electrode or by a ground electrode) and measuring the voltage on the signal input during the current flow. From the measured voltage, one can determine the impedance of the line, to which are added the impedance of the electrodes and the impedance of the body of the patient between the two electrodes included in the current loop.
One can thus highlight any defects of the electric conduction within the cables, as well as any defective connection of the electrodes (e.g., a badly inserted or loose cable or electrode, an internal break, etc.) by thus measuring, in place, the impedance of the line, and, in the case of a Holter recorder, of the lines of the various ECG cables.
The impedance measurement can be carried out not only after installation of the electrodes at the time of putting the device in service, but also throughout the duration of the recording duration: Indeed, it is possible to detect a detachment of an electrode, the disconnection of a cable, etc., and to take account of this defect when recording signals or in the analysis of the acquired signals, or to even ask the patient being monitored to restore continuity by replacing the electrode (or cable) or by replugging a disconnected cable.
The small current used in the impedance measurement is typically a microampere-current (“micro-current”) and is preferably of an impulse (pulsitile) nature. This is because a continuous micro-current would be likely to cause a polarization of the electrode-skin couple, and an alternating micro-current would involve a relatively significant consumption of energy because of the high frequency to be used, about 100 KHz, which must be higher than the components of the ECG signal (including the peaks of the stimulation pulses generated by a pacemaker eventually implanted in the patient), and of the signals which must be rejected.
For this reason, the most recent apparatuses, such as the Syneflash and Synesis model recording devices available from Ela Medical, Montrouge, France, implement such an impulse process, in which the device injects a small quantity of current (on the order of a microampere) during a very short time (of about a microsecond) to measure the impedance of a line. The amplifier on which the input is connected amplifies the variation of voltage caused by the injection of this current and retransmits this variation to a digital measurement system (analog/digital converter and microcontroller) which memorizes and processes the impedance data, without (theoretically) deterioration of the collected signal.
This process has been found to present two disadvantages, however, in certain situations. First, in the case when an electrode is disconnected, the voltage level at the input on which the current is injected is parasitized (a situation that results from an illegible signal or an interference at the frequency of the power line), or falls to zero. If the level falls to zero, the device receives in fact on the amplifier the ECG signal of the other electrode. It thus amplifies this other signal in a monopolar mode, i.e., compared to the patient ground, thus giving a signal which seems very correct, although detected as abnormal by the microcontroller. In this situation, the system of current injection presents a major defect due to the fact that the load injected on the line which has a high impedance (because of the break in or disconnection of the cable). This impedance creates a voltage that is maintained for a relatively long time because of the input capacitors of the sensing circuit (these capacitors being useful for the passive filtering of the high frequencies). The sensed ECG signal is then very strongly disturbed by peaks caused with each injection of micro-current.
Second, in the case of a patient carrying a pacemaker, the stimulation pulses (or peaks) produced by the implanted device can obstruct the impedance measurement of the external recorder, in two respects. First, if the stimulation peak occurs at the same time that measurement is taken, it can considerably deform the response and give a false value of the impedance. Second, when the voltage peak caused by the injection of current is significant, it can be wrongly interpreted as a stimulation peak by the recorder.